1. Field of the Invention
The invention is directed to methods and apparatus for recovering representative connate samples taken from low permeability, high pressure formations.
2. Description of the Prior Art
A goal of earth formation testing is to obtain representative samples of fluids extracted from the formation. In order to control the pressure drop from the formation pressure to the sample chamber, previous techniques for extracting a fluid sample include the use of passive, fixed chokes to meter water exiting from the back side of a piston while the sample is received on the top. Assignee's Modular Dynamics Formation Tester, also known as the MDT, a mark of Schlumberger, uses throttle valves at the sample chamber inlet to control pressure drawdown in the flowline. However, the MDT does not control the pressure once the fluid passes the throttle valve. The sample chamber with the throttle valve is also used as the water receiver for multi-sample bottles to control the flow rate; however, this combination is unable to achieve sufficiently low flowrates to prevent flow line pressure from dropping below the bubble point.
Reservoir engineers use wireline formation testers to obtain pressure profiles, formation fluid samples and permeability indicators--information that is crucial during exploration and development of oil and gas fields. Wireline formation testers have been used to collect formation fluid samples. The instant invention improves the quality of samples by using techniques and apparatus for downhole fluid analysis which include: a system for discarding contaminated fluids before taking samples, identifying the fluid flowing from the formation and limiting the drawdown pressure by using precision flow control methods, i.e., very low flow rates (e.g., 1 cc/sec) at very high pressures (up to 20,000 psi).
Proper reservoir management requires formation pressure measurements in a wide range of conditions. Formation pressure measurements taken within a well can be plotted versus true vertical depth to produce a pressure profile. Formation pressure is obtained by withdrawing a small amount of fluid from the formation to generate a short transient test. The pressure response is then recorded during shut-in until it stabilizes.
A major problem which occurs in low-permeability reservoirs is the relatively high drawdown that occurs during sampling. In some cases, this drawdown lowers the flowing pressure below the bubblepoint pressure at which gas is liberated from the solution, thereby producing an unrepresentative sample. In gas condensate reservoirs, the sampling pressure may be lower than the dew point pressure and liquid may remain in the formation while the gas escapes, again providing a nonrepresentative sample. The instant invention offers surface control of drawdown pressure, a feature that allows pretests and sampling in tight reservoirs and avoids large drawdowns.
A major difficulty that commonly occurs when sampling with wireline testers is contamination by deep filtrate invasion during drilling. In the instant invention, properties of the fluid entering the flowline are constantly monitored from the surface using resistivity or temperature and/or optical properties measurements. The formation fluid that enters the tools is returned to the wellbore until a sample is judged to be representative of the formation. This desired fluid sample is then diverted into the sample chambers, allowing the collection of high-quality samples. Furthermore, several samples can be taken during a single run into the hole, thus, several zones can be sampled in a single trip, resulting in rig-time savings.
For the foregoing reasons, there is a need for an apparatus and method which provides a high quality, representative formation sample at a low sample rate in a high pressure, low permeability well.